The end of life section is a bit whitewashed. Freeman Dyson had an article (struggling to find it) about how von Neumann was invited to give a Hilbert-style lecture on the 10 problems for the new computing age. All the big names at the time were there, but von Neumann was in such poor shape (dementia I assume), that it was decided to pretend it had never happened.

edit: article is called Birds and Frogs, by Freeman Dyson https://wucj.lab.westlake.edu.cn/Others/Dyson_Birds_and_Frog...

He died from cancer - he was only in his 50's. Unlikely to be dementia.